The present invention relates to a step-by-step variable transmission, in particular for motor vehicles, having a housing, having a drive input shaft device, having a drive output shaft and having a countershaft, with a plurality of gearwheel sets being mounted on the drive input shaft and on the countershaft, with it being possible for the gearwheel sets to be shifted so as to establish different gear stages by means of respective shift clutches, with the countershaft being connected to the drive output shaft by means of a drive output constant gearwheel set, and with the drive output shaft being rotatably mounted in the region of its opposite ends on the housing by means of two bearings.
A step-by-step variable transmission of said type is known from EP 1 031 765 A2. Said known transmission is a so-called in-line transmission, that is to say a transmission for longitudinal installation in the motor vehicle.
Conventional in-line transmissions of said type generally have a drive input constant gearwheel set. A drive input constant gearwheel set of said type usually comprises, at the input of the transmission, a fixed gearwheel which is fixedly connected to the drive input shaft and a fixed gearwheel which is fixedly connected to the countershaft. In the case of dual clutch transmissions, a separate drive input constant gearwheel set is usually provided for each drive input shaft.
A drive input constant gearwheel set of said type establishes a ratio for the transmission-input-side torque (generally the torque of an engine of the motor vehicle such as an internal combustion engine) directly to the transmission input. This leads to a comparatively high torque loading at the countershaft.
This has the result that the toothings of the gearwheel sets for the different gear stages must be designed to be relatively wide in order to withstand the high torques. The shift clutches must also be designed for the comparatively high torques, in particular when the shift clutches are, as is conventional, so-called synchronous clutches.
In contrast, EP 1 031 765 A2 as cited in the introduction relates to an in-line transmission with a drive output constant gearwheel set. In such a transmission type, a constant gearwheel set is seated in the region of the transmission output or in the region of the drive output shaft. As a result of this measure, the torque loading at the countershaft is reduced. However, the rotational speed level in the region of the countershaft is increased in relation to solutions with a drive input constant.
As a result of the relatively low torque loadings, it is possible for the toothings of the gearwheel sets to be of narrower design than in the case of solutions with a drive input constant. The transmission can therefore generally be of shorter design in the axial direction. Only the drive output constant gearwheel set must be of considerably wider design than in the case of step-by-step variable transmissions with a drive input constant gearwheel set. The loading of the shift clutches (synchronizations) is also reduced. It is hereby also possible to obtain shorter shift times.
One problem of step-by-step variable transmissions with a drive output constant gearwheel set is that it is comparatively difficult to mount the drive output constant gearwheel set in such a stiff manner that it still has an acceptable contact pattern despite its comparatively large width. Although the drive output shaft can generally be of short design from conception, for said reason, it is nevertheless usually mounted so as to project far back, as is described for example in EP 1 031 765 A2.
The conceptually obtainable installation length advantage of the transmission type is therefore relinquished again.
DE 39 32 264 C2 discloses a further step-by-step variable transmission with a drive output constant. The transmission is embodied as a 5-gear transmission, with the fourth gear being designed as a direct gear in which the drive input shaft is connected directly to the drive output shaft. In addition, the drive input shaft is mounted in the drive output shaft. Here, too, the drive output shaft projects relatively far out in the axial direction.
A further transmission concept with a drive output constant is known from EP 0 798 491 B1. This is a 6-gear transmission in which the fourth gear stage is embodied as a direct gear stage. The drive input shaft is again mounted in the drive output shaft. A complex mounting of the drive output shaft is provided in this embodiment also.